1. Technical Field
Embodiments of the invention relate to a planar transformer and/or transmission line balun structure having a small trace spacing and high mutual coupling coefficient and, to a method of fabricating the planar transformer or balun structure.
2. Description of Related Art
Currently, planar transformers are fabricated on single substrate, where both primary and secondary windings or traces of the transformer are monolithically built on the same substrate. To ensure high current-carrying capability and high quality factor, the trace thickness has to be sufficiently large. Large trace thickness, coupled with process limitations, results in large trace spacing and ultimately a large package form factor. For example, in a package substrate, a trace thickness of between 20 μm to 30 μm requires a minimum trace spacing of about 85 μm. On a die substrate, e.g. gallium arsenide or glass, a trace thickness of about 60 μm requires a trace spacing of about 30 μm.
Planar transformers are utilized in wireless communication devices including, but not limited to, transformer-based baluns to convert signals between differential and single-ended modes, for signal filtering in band-pass filters or balanced diplexers, and in differential circuits such as mixers and voltage controlled oscillators. In these various circuits, the transformer can be used, amongst others, for signal balancing, DC isolation or impedance matching. Additionally, in computing systems, transformers may be used in power delivery applications such as coupled buck voltage regulators. In such applications, high quality factor is desired to reduce losses. Also, strong electromagnetic coupling between the primary and the secondary windings of the transformer is desired to provide strong signal transmission therebetween. However, large trace spacing is a severe limitation to increasing electromagnetic coupling and reducing package form factor.